In a wireless system, such as in LTE (Long Term Evolution), a decoder is commonly used for correcting bit errors which have occurred during data transmission over the physical layer (also named L1 or the air interface). A common situation when bit errors may occur is when the received signal of interest is weaker than the surrounding noise and interference. The signal may, for example, be so weak that it is drowning in the thermal noise generated by different electrical components in the radio receiver, such as the resistors and transistors. Another common source of bit errors is when transmissions from neighboring LTE cells are leaking in and interfere with the signal of interest.
Different kinds of decoders exist, such as convolutional, turbo, block etc. Common to all these decoders is that their performance is improved if soft information is used at their input. The bit soft values input to the decoder are most commonly defined as the log-likelihood probability ratio of having received a bit as either zero or one. The sign of the bit soft value determines if the bit is a zero or a one and its magnitude determines the certainty of this decision. A bit soft value with a large magnitude is considered very reliable, while a bit soft value with a magnitude of zero is totally unreliable. Further details on bit soft value computation may be found in, for example, [1].
The received modulated data symbols (sometimes also referred to as modulation symbols) are often enhanced in an equalizer. The main purpose of the equalizer is to remove Inter-Symbol Interference (ISI) of the received signal. Inter-symbol interference is generated by delayed echoes of the original signal caused by the signal first being reflected on distant objects before reaching the receiver antenna.
After possible equalization a step called soft demapping is used. In this step each equalized symbol is converted into a sequence of log-likelihood ratios or bit soft values. The bit soft values are calculated by looking at the distances between the (possibly equalized) modulated data symbol and each point in the constellation diagram of the used modulation scheme.
An important part of the generation of the bit soft values is to normalize them properly. This normalization factor is inherent in the definition of the log-likelihood ratio. In the derivation of the log-likelihood ratios it turns out that the noise power in the received signal can be treated as a normalization factor. Hence, by using a good estimation of the noise power a good normalization of the bit soft values can be achieved.
In LTE the noise power can be estimated over the Reference Signal (RS). However, a problem with an RS based approach is that it only works well as long as the noise and interference is stationary over a complete slot. If the noise or interference level changes somewhere outside of the RS, the noise estimation corresponding to this time region will be incorrect and the bit soft values will get an incorrect normalization.